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In 2007, two super-Earths known as Gliese 667 C and Gliese 581d were discovered orbiting red dwarfs in the habitable zone, an area in which a planet is able to have surface temperature in order to liquid water. Recently, results from a study suggest these planets plus smaller, rocky ones are quite common in our galaxy and orbit red dwarfs by the tens of billions.
The study was conducted by an international team of scientists a part of the HARPS (High Accuracy Radial velocity Planet Search), ESO’s (European Southern Observatory) planet finder. HARPS’s mission is to detect planets beyond the solar system. HARPS especially aims to discover planets that are in the habitable zone.
In order to calculate the largest amount of Earth-like planets that could exist in the Milky Way, HARPS studied the most common type of star in the galaxy: red dwarfs. Red dwarfs are small, cool, and faint in luminosity in comparison to the Sun. Because they spend less energy than other types of stars, they are long-lived and, therefore, are the most common. Approximately 160 billion exist in the galaxy alone, making up a whopping 80% of the total number of stars.
Using a spectrograph from a 3.6-meter telescope from La Silla Observatory in Chile, HARPS chose a sample of 102 red dwarfs from the southern portion of the sky and studied them for six days. HARPS detected nine super-Earths (planets up to ten times the size of the Earth), two of which were inside the habitable zone. Furthermore, 40% of red dwarfs contain super-Earths that are able to sustain water on their surfaces.
Combining their data and the number of stars without planets, and an estimate of how many planets could be discovered, HARPS was then able to calculate the total number of planets orbiting red dwarfs and the different types of these planets. In the end, their results illustrated that tens of billions of smaller rocky planets exist in the Milky Way.
100 of these hypothesized planets should exist in the immediate vicinity – around 30 light-years – of the Sun (smaller planets are difficult to detect). Massive gassy planets (around the size of Jupiter and Saturn), on the other hand, were calculated to be rare when it came to orbiting red dwarfs.
Although it is exciting knowing that so many Earth-sized orbit stars in the habitable zone, astronomers are not getting their hopes up of finding life. It would be difficult for life to thrive on planets that orbit red dwarfs: because red dwarfs are cool, the habitable zone is rather close, leaving any planets close to the red dwarf to be bombarded with flares of ultraviolet rays and X-rays, making the planets not habitable after all. But that does not daunt astronomers of thinking that any of these small worlds could harbor life.
“Now that we know that there are many super-Earths around nearby red dwarfs,” Xaiver Delfosse, a member of the team tells ESO, “we need to identify more of them using both HARPS and future instruments. Some of these planets are expected to pass in front of their parent star as they orbit — this will open up the exciting possibility of studying the planet’s atmosphere and searching for signs of life.”
A detailed report of HARPS experiment and results can be found here.